linux-kernel - Re: [QUESTION] problems report: rcu_read_unlock_special() called in irq

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Message-ID: <aGUT4sCXE_56Wsv-@localhost.localdomain>
Date: Wed, 2 Jul 2025 13:11:30 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Qi Xi <xiqi2@...wei.com>, paulmck@...nel.org,
	Xiongfeng Wang <wangxiongfeng2@...wei.com>,
	Joel Fernandes <joel@...lfernandes.org>, ankur.a.arora@...cle.com,
	Boqun Feng <boqun.feng@...il.com>, neeraj.upadhyay@...nel.org,
	urezki@...il.com, rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Wangshaobo (bobo)" <bobo.shaobowang@...wei.com>,
	Xie XiuQi <xiexiuqi@...wei.com>
Subject: Re: [QUESTION] problems report: rcu_read_unlock_special() called in
 irq_exit() causes dead loop

Le Wed, Jul 02, 2025 at 06:59:38AM -0400, Joel Fernandes a écrit :
> 
> 
> On 7/2/2025 5:14 AM, Qi Xi wrote:
> > Hi Joel,
> > 
> > After applying the 2 patches, the problem still exists. Compared to the previous
> > fixes which did solve the problem, the difference is ct_in_irq() in the first
> > patch.
> > 
> > I am wondering why "nesting != CT_NESTING_IRQ_NONIDLE" is added?
> > 
> > 
> > (previous fix: problem is solved)
> > 
> > +bool ct_in_irq(void)
> > +{
> > +    return ct_nmi_nesting() != 0;
> > +}
> > 
> > (current fix: problem still exists)
> > 
> > +bool ct_in_irq(void)
> > +{
> > +    long nesting = ct_nmi_nesting();
> > +
> > +    return (nesting && nesting != CT_NESTING_IRQ_NONIDLE);
> > +}
> 
> Oh gosh, thanks for spotting that! Indeed,  I had changed it to != 0 in the last
> version but applied an older patch. I will fix it in the tree. Thank you again!
> 
> Neeraj, would you like this as a separate commit that you can then squash? Or
> could you fix it up in your tree?

nesting == CT_NESTING_IRQ_NONIDLE means that the rcu_is_watching() and we are
not in an interrupt. We still need to rule out that case, right?

Is there something else that makes it not working?

-- 
Frederic Weisbecker
SUSE Labs